<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>Simple Adder</title>
    <style>
        body{
            min-width: 400px;
        }
        input[type=text] {
            box-sizing: border-box;
            width: 100px;
            height: 30px;
            text-align: center;
            font-size: 16px;
        }

        input[type=text]:focus {
            outline: none;
        }

        label {
            cursor: pointer;
        }
    </style>
</head>
<body>

<form action="#">
    <input type="text" id="augend" autofocus>
    +
    <input type="text" id="addend">
    =
    <input type="text" id="sum">

    <br>
    <br>

    <label for="instant">
        <input type="checkbox" id="instant" checked>
        output sum immediately
    </label>
</form>
<script src="native-implementation.js"></script>
</body>
</html>